So called Merlin type pipe connectors are well known in the art for connecting pipes together and are disclosed in GB1573945, GB2033518, GB2099529, GB2113335 and GB2138089. The connection is formed by tubular pin member having a frustoconical outer peripheral surface and a tubular box member having a generally frustoconical inner peripheral surface corresponding to the frustoconical outer peripheral surface of the pin member. In use, the two members, each associated with a pipe section, are telescoped together and are axially locked together by mating annular projections and grooves provided on the said peripheral surfaces, the projections and grooves being spaced apart along the two surfaces.
In telescoping the two members together, they are initially telescoped until surface contact is made between crest surfaces of the projections and surfaces between the grooves at least at the ends of the overlapped portions of the surfaces. Hydraulic fluid under pressure is then typically supplied between the overlapped parts of the surfaces to expand the box member and/or contract the pin member to permit the members to be fully telescoped together or the members may simply be pushed together. Pressurised hydraulic fluid is also used to disengage the members by expanding the box member and/or contracting the pin member to bring the projections out of engagement with the corresponding grooves.
In order to reduce the axial extent of the members, projections and grooves may be provided which have relatively small axial extents but this means that, to fully telescope the members together after they have been brought into initial contact, it is necessary to move individual projections past at least one groove before each projection is aligned with the groove in which it is designed to engage. In doing this, there is a risk that the projections and grooves may inter-engage before the members are fully telescoped together and it may then prove impossible to disengage the projections and grooves. To avoid this, prior are systems have proposed forming some at least of the projections and grooves with different axial extents so that the projections and grooves cannot inter-engage before the members are fully engaged. The arrangement is such that in all intermediate positions of the members, prior to full engagement and after the frustoconical surfaces have been brought into initial surface contact, there is contact between the crests of at least some of the projections and surfaces between the grooves spaced apart along the overlying parts of the frustoconical surfaces.
These prior art connectors have the problem, however, that when disengaging the members using pressurised hydraulic fluid, to ensure that the projections and grooves at the end of the surfaces of the members overcome the radial restrain and disengage completely, the radial loading imposed by the pressurised fluid on the central portion of the surfaces of the members is substantially greater than is needed to disengage the projections and grooves in these sections. This can result in the elastic limit of the material of the connectors parts in these sections being exceeded, permanently deforming the parts and hence preventing re-use. This problem has been overcome by making the members out of high tensile steel, but this significantly increases cost.
Applicant's own earlier European patent EP 0803637 discloses a solution to this problem in which the radial height of the projections is reduced towards the free end of each member so that the engagement depth of those projections in the grooves is reduced. In particular, the radial height of the projections taper inward towards the free end of each member. This decreases the depth of engagement of the teeth in the grooves, so as to provide sufficient clearance of the end teeth during break-out to enable disengagement at a lower pressure while keeping the average tooth engagement relatively high. In this way, the risk of over-pressurising the members beyond the elastic limit of the material in the middle of the connector is reduced.
The system has the drawback, however, that the teeth crests are no longer aligned on a single cone and uneven crest profile causes contact pressure high points during assembly, which can lead to excessive local wearing of the teeth. Moreover, tapering the teeth crests at the ends compromises the metal-metal seal between the end teeth during make-up of the joint, meaning that a high viscosity injection fluid is required to enable the high injection pressure required for a smooth make-up.